Method of operation of a cell for recovery of aluminium byelectrolysis of aluminium oxide in a fluoride melt

ABSTRACT

A method of operation of a cell for recovery of aluminium by electrolysis of aluminium oxide in a fluoride melt, in which the value of electrical energy supplied to the cell is ascertained over successive intervals of time and each value is compared with a desired value of the electrical energy supply for the same interval based upon a desired rate of energy supply, and any difference between desired and actual value for each interval is added, with appropriate sign, to a cumulative total, and when the cumulative total exceeds a predetermined value then the average interpolar distance is increased or decreased in the sense tending to establish a rate of energy supply more nearly equal to the desired rate.

United States Patent Chaudhuri et al.

[ Jan. 7, 1975 METHOD OF OPERATION OF A CELL FOR RECOVERY OF ALUMINIUM BYELECTROLYSIS OF ALUMINIUM OXIDE IN A FLUORIDE MELT [75] Inventors: Kiranendu Chaudhuri, Gampel;

Peter Bachofner, Liebefeld, both of [21] Appl. No; 378,032

[30] Foreign Application Priority Data July 18, 1972 Switzerland 10749/72 [52] US. Cl. 204/67, 204/225 [51] Int. Cl C22d 3/12, BOlk 3/00 [58] Field of Search 204/67, 225

[56] References Cited UNITED STATES PATENTS 3,674,674 7/1972 Arts et al. 204/225 3,76l,379 9/1973 Elliott 204/225 FOREIGN PATENTS OR APPLICATIONS 188,678 ll/l966 U.S.S.R 204/67 [929,262 2/1970 Germany 204/67 302,39l 4/1968 U.S.S.R 204/225 Primary Examiner-John H. Mack Assistant ExaminerD. R. Valentine Attorney, Agent, or Firm-Ernest F. Marmorek [57] ABSTRACT A method of operation of a cell for recovery of aluminium by electrolysis of aluminium oxide in a fluoride melt, in which the value of electrical energy supplied to the cell is ascertained over successive intervals of time and each value is compared with a desired value of the electrical energy supply for the same interval based upon a desired rate of energy supply, and any difference between desired and actual value for each interval is added, with appropriate sign, to a cumulative total, and when the cumulative total exceeds a predetermined value then the average interpolar distance is increased or decreased in the sense tending to establish a rate of energy supply more nearly equal to the desired rate.

2 Claims, 1 Drawing Figure lllllllllllllll lllllllllllllllllll llllllllllllllllllllllllllllllllllll llll METHOD OF OPERATION OF A CELL FOR RECOVERY OF ALUMINIUM BYELECTROLYSIS OF ALUMINIUM OXIDE IN A FLUORIDE MELT For the recovery of aluminium by electrolysis of aluminium oxide (A1 alumina) the latter is dissolved in a fluoride melt, which consists in the greatest part of cryolite Na AlF This melt is contained in a cell having a carbon bottom. The aluminium separated at the cathode collects in liquid state on the carbon bottom of the cell beneath the fluoride melt, and the upper surface of this liquid aluminium in fact constitutes the cathode. Anodes of amorphous carbon dip from above into the melt. Oxygen arises at the anodes by the'electrolytic decomposition of the aluminum oxide, and combines with the carbon of the anodes to CO and C0 The electrolysis takes place in a temperature range of about 940 to 975 C.

The object of the present invention is to control the heat production in a cell for recovery of aluminium by electrolysis of aluminium oxide in a fluoride melt, that is to say to hold it to an optimum desired value.

The principle of an aluminium electrolysis cell with pre-baked anodes appears from the single FIGURE of the accompanying drawing. This shows a vertical section in the longitudinal direction through part of a known electrolysis cell.

The steel shell 12, which is lined with a thermal insulation 13 of heat-resisting, heat-insulating material and with carbon 11, contains the fluoride melt (the electrolyte). The aluminium 14 separated at the cathode lies on the carbon bottom 15 of the cell. The surface 16 of the liquid aluminium constitutes the cathode. In the carbon lining 11 there are inserted iron cathode bars 17 transverse to the longitudinal direction of the cell, and these conduct the electrical direct current from the carbon lining ll of the cell laterally outwards. Anodes 18 of amorphous carbon dip from above into the fluoride melt l0, and supply the direct current to the electrolyte. They are firmly connected via conductor rods 19 and clamps 20 with the anode beam 21. The current flows from the cathode bars 17 of one cell to the anode beam 21 of the following cell through conventional current bus bars, not shown. From the anode beam 21 it flows through the conductor rods 19 the anodes 18, the electrolyte 10, the liquid aluminium 14, and the carbon lining 11 to the cathode bars 17. The elctrolyte 10 is covered with a crust 22 of solidified melt and there is a layer of aluminium oxide 23 lining above the crust. In operation, cavities 25 occur between the electrolyte l0 and the solidified crust 22. Against the side walls of the carbon lining 11 there likewise forms a crust of solid electrolyte, namely a lateral ledge 24. The horizontal extent of the lateral ledge 24 affects the plan area of the bath of liquid aluminium 14 and electrolyte 10.

The distance d from the lower side 26 of the anode to the surface 16 of the liquid aluminium, also known as the interpolar distance, can be adjusted by lifting or lowering of the anode beam 21 with the help of the lifting mechanism 27, which is mounted on pillars 28. This effects all the anodes. An anode can be adjusted individually by releasing the respective clamp 20, shifting the respective conductor rod 19 upwards or downwards relatively to the anode beam 21, and retightening the clamp.

Because of attack by the oxygen released during electrolysis, the anodes are consumed continuously on their lower side, by about 1.5 to 2 cms per day according to the type of cell. At the same time, the height of the liquid aluminium on the bottom of the cell increases continuously by about 1.5 to 2 cms per day due to the alu minium separated at the cathode.

When an anode has been consumed, then it is exchanged for a fresh anode. In practice, the cell is operated in such a way that, some days after its start of use, the anodes of the cell no longer have the same degree of consumption, and therefore they must be exchanged separately over a range of several weeks. For this reason, anodes of different starting dates operate together in the same cell, as appears from the drawing.

Because of this complex situation, the interpolar distances d of individual anodes are not exactly equal to each other. It suffices for the purpose of the present invention to consider the average, at any moment in time, of the individual interpolar distances. This average interpolar distance, which itself varies with time, will be termed D.

The principle of an aluminium electrolysis cell with one or more self-baking anodes (Soederberg anodes) is the same as that of an aluminium electrolysis cell with pre-baked anodes. Instead of pre-baked anodes, one or more anodes are used which are continually baked from a green electrode paste in a steel jacket during the electrolytic operation by the heat of the cell. The direct current is supplied by lateral steel rods or from above by vertical steel studs. These anodes are renewed as required by pouring green electrode paste into the steel jackets. Adjustments of interpolar distance are made by vertical adjustments of the steel jacket.

By breaking in of the upper electrolyte crust 22 (the crusted bath surface), the aluminium oxide 23 which is above it is brought into the electrolyte 10. This operation is known as servicing of the cell. In the course of the electrolysis, the electrolyte becomes depleted in aluminium oxide. When the concentration of aluminium oxide in the electrolyte falls to somewhere between 1 and 2%, there arises the anode effect, which results in a sudden increase in cell voltage from the normal 4 to 4.5 volts to 30 volts and above. Then at the latest the crust must be broken in, and the A1 0 concentration be raised by addition of new aluminium oxide.

The aluminium 14 produced electrolytically, which collects on the carbon bottom 15 of the cell, is generally removed once a day from the cell by conventional tapping devices, for instance sucking devices.

One measurable quantity in the operation of the cell is its base voltage. This depends on the age of the cell, the condition of the carbon lining 11, and the composition of the molten electrolyte 10, as well as on the cell current intensity and current density. The base voltage is also affected by the variation of the plan area of the bath in consequence of variation of the horizontal extent of the lateral ledge 24. The base voltage is measured between corresponding points on the anode beams of the cell in question and of the next cell in series. The voltage is the total of the ohmic voltage drops in the parts of the cell through which current flows plus the EMF required for the electrolytic decomposition of the A1 0 in the electrolyte.

The average interpolar distance is the determining factor in the heat produced in the cell. That is to say, at constant current, variation of the average interpolar distance D causes variation in the cell base voltage and hence in the energy supplied to the cell, and variation in energy supplied involves a variation in heat produced. Thus an optimum value of the production of heat is obtained if the average interpolar distance is at an optimum value. If too little energy is supplied to the cell, the temperature of the electrolyte begins to drop. The consequences then include too thick lateral ledges with reduction of the bath plan area, and formation of troublesome sludge on the carbon bottom by separation of solid components from the electrolyte. lf, on the contrary, too much energy is supplied to the cell, the temperature of the electrolyte rises, the lateral ledges melt with increase of the plan area of the bath, and the current efficiency and the specific electrical energy consumption worsen.

In practice, the actual average interpolar distance is sometimes larger or smaller than the optimum average interpolar distance. The departures are substantially produced by increase of the height of the liquid aluminium 14 above the carbon bottom 15, and by burning away of the anodes 18 at their lower side 26 at well as by variations of the thickness of the lateral ledges of frozen electrolyte. Differences of the interpolar distance of individual anodes can be caused by defective insertion of new anodes during anode exchange, slipping of the conductor rods 19 as a result of insufficient tightening of the clamps 20, unequal anode quality, bulging ofthe surface 16 of the liquid aluminium 14 as a consequence of magnetic effects.

According to the present invention, in the operation ofa cell for recovery of aluminium by electrolysis of aluminium oxide in a fluoride melt, the value of electrical energy supplied to the cell is ascertained over successive intervals of time and each value is compared with a desired value of the electrical energy supply for the same interval based upon a desired rate of energy supply, and any difference between desired and actual value for each interval is added, with appropriate sign, to a cumulative total, and when the cumulative total exceeds a predetermined value then the average interpolar distance is increased or decreased in the sense tending to establish a rate of energy supply more nearly equal to the desired rate.

The desired rate of electrical energy supply should be determined separately for each cell type and each cell. It must correspond to the condition and the age of the cell. The desired rate of electrical energy supply is for each type of cell and for each cell an empirical value, which leads to an optimum overall cell performance. One may revise the value of the desired rate of electrical energy supply about once every week, depending on the overall cell conditions (for instance cathode voltage drop, ledge conditions, sludge on the bottom of the cell and so on).

In practice the arithmetical operations may be carried out by a computer.

When the cumulative total exceeds the predetermined value, it is desirable to check that no individual anode has a notably smaller interpolar distance d than the remainder. if such a condition exists, then it is best to adjust the individual anode, and not to adjust the remainder at that time.

If an anode effect occurs within a measuring interval, the additional energy supplied during the anode effect will be omitted from the cumulative total ofelectric energy or the predetermined value will be changed accordingly.

Whenever an adjustment of the average interpolar distance is made, whether by adjusting all the anodes or a single anode, then the cumulative total is cancelled, and the procedure starts again from a cumulative total of zero.

In more detail, starting from a datum point in time, measurements are preferably made of cell base voltage and of cell current at uniform intervals of time. After each interval, the product of voltage times current times length of interval, is compared with the value of desired energy per interval, and the difference, if any is added (with positive or negative sign as appropriate) to a cumulative total. If at any time the cumulative total difference exceeds a predetermined value, then, unless an individual anode requires to be adjusted, the average interpolar distance is altered, in the appropriate sense, by raising or lowering the anode beam through a predetermined linear step.

For example, with a particular cell designed to operate at a working current intensity of kA, the optimum base voltage amounts to 4.2 volts, which corresponds to a base cell resistance of 25.5 microhms. At a testing interval of 50 seconds the desired energy per interval consequently amounts to 5.85 kWh. If the eumulative total of differences between measured energy and desired energy exceeds 10 kWh, the average interpolar distance is altered by a value which corresponds to a resistance alteration of 1 microhm, that is to say an alteration of the cell voltage of about 0.1 volts. This is eqivalent to a movement of the anode beam of about 3 mm, in the case of a cell which has an anodic current density of about 0.8 amps/cm? The normal average interpolar distance of a 100 kA cell lies around 5 to 5.5

The advantage of the method according to the invention lies in the maintenance of narrow limits for the resistance of the cell, by which excessive heating and excessive cooling of the electrolyte are both counteracted. The consequences are an improvement of the current efficiency and reduction of the specific electrical energy consumption.

What we claim is:

1. A method of operation of a cell having an anode and means for changing the average interpolar distance, for recovery of aluminium by electrolysis of aluminium oxide in a fluoride melt, comprising measuring values of electrical energy supplied to the cell at respective successive predetermined intervals of time, comparing each of said measured values to a corresponding predetermined value of electrical energy supply for the respective interval of time, said corresponding predetermined value being based upon a predetermined rate of energy supply, adding successively any difference between each of said measured and said corresponding predetermined values with appropriate sign, to obtain a cumulative total, comparing said cumulative total to predetermined lower and upper limits, and changing the average interpolar distance to establish an energy supply more nearly equal to the predetermined value.

2. The method according to claim 1, in which there are a plurality of anodes and the change in the average interpolar distance is produced by moving all of the anodes of the cell by predetermined linear step. 

1. A METHOD OF OPERATION OF A CELL HAVING AN ANODE AND MEANS FOR CHANGING THE AVERAGE INTERPOLAR DISTANCE, FOR RECOVERY OF ALUMINIUM BY ELECTROLYSIS OF ALUMINIUM OXIDE IN A FLUORIDE MELT, COMPRISING MEASURING VALUES OF ELECTRIAL ENERGY SUPPLIED TO THE CELL AT RESPECTIVE SUCCESSIVE PREDETERMINED INTERVALS OF TIME, COMPARING EACH OF SAID MEASURED VALUES TO A CORRESPONDING PREDETERMINED VALUE OF ELECTRICAL ENERGY SUPPLY FOR THE RESPECTIVE INTERVAL OF TIME, SAID CORRESPONDING PREDETERMINED VALUE BEING BASED UPON A PREDETERMINED RATE OF ENERGY SUPPLY, ADDING SUCCESSIVELY ANY DIFFERENCE BETWEEN EACH OF SAID MEASURED AND SAID CORRESPONDING PREDETERMINED VALUES WITH APPROPRIATE SIGN, TO OBTAIN A CUMULATIVE TOTAL, COMPARING SAID CUMULATIVE TOTAL TO PREDETERMINED LOWER AND UPPER LIMITS, AND CHANGING THE AVERAGE INTERPOLAR DISTANCE TO ESTABLISH AN ENERGY SUPPLY MORE NEARLY EQUAL TO THE PREDETERMINED VALUE.
 2. The method according to claim 1, in which there are a plurality of anodes and the change in the average interpolar distance is produced by moving all of the anodes of the cell by predetermined linear step. 